The present invention relates to new antidiabetic compounds and, in particular, to novel 2-substituted-5′-O-(1-boranotriphosphate)-adenosine derivatives which are potent and selective insulin secretagogues.
Pathophysiology of Diabetes Mellitus
Diabetes mellitus is one of the most prevalent chronic diseases in the Western world, affecting up to 5% of the population. It is a heterogenous group of disorders characterized by a chronic hyperglycemia with additional abnormalities in lipid and protein metabolism. The hyperglycemia results from defects in insulin secretion, insulin action, or a combination of both. In addition to its chronic metabolic abnormalities, diabetes is associated with long-term complications involving various organs, especially the eyes, nerves, blood vessels, heart and kidney, which may result in blindness, amputations, cardiovascular disease and end stage renal disease. The two major forms of diabetes are classified as type 1 and type 2. Type 2 diabetes, previously termed non-insulin-dependent diabetes mellitus (NIDDM), is the most prevalent form of the disease, affecting approximately 95% of patients with diabetes.
Type 2 Diabetes Mellitus
The development of diabetic complications appears to be related to the chronic elevation of blood glucose. There is no current cure for diabetes, however, effective glycemic control can lower the incidence of diabetic complications and reduce their severity.
Type 2 diabetes appears to be a complex polygenic disease in which insulin resistance and relative insulin deficiency coexist. Thus, improvement of insulin secretion is a major therapeutic goal. The deficiency of insulin release expresses itself not only by the absence of first-phase insulin response to glucose, but also by a global reduction in the magnitude of insulin release to 10-20% of the normal secretory capacity (Cerasi, 1992).
Treatment of Hyperglycemia in Type 2 Diabetes Mellitus
Patients with type 2 diabetes are treated with various oral antidiabetic agents, insulin injections, or a combination of both. The currently available oral antidiabetic drugs are targeted at either reducing peripheral insulin resistance, increasing insulin secretion from the pancreatic beta-cell, or slowing the absorption of carbohydrates from the intestine.
Approximately half of the patients with type 2 diabetes are treated with oral agents, a considerable proportion of them with agents that stimulate insulin secretion. The choice of insulin secretagogues is limited to the sulfonylureas and related compounds (“glinides”), which elicit insulin secretion by binding to a regulatory subunit of membrane ATP-sensitive potassium channel, inducing its closure (Lebovitz, 1994). Two types of agents are used to attenuate peripheral insulin resistance: the biguanide metformin and the thiazolidinedione analogues (Edelman, 1998). The α-glucosidase inhibitor, pseudotetrasaccharide acarbose, is used to slow intestinal absorption of carbohydrates.
Sulfonylureas have several undesired effects in addition to possible long-term adverse effect on their specific target, the pancreatic beta-cell. These side-effects include the risk of hypoglycemia due to stimulation of insulin secretion at low glucose concentrations, the difficulty of achieving normal glycemia in a significant number of patients, the 5-10% per year secondary failure rate of adequate glycemic control, and possible negative effects on the cardiovascular system (Lebovitz, 1994; Leibowitz and Cerasi, 1996; Brady and Terzic, 1998).
P2-Receptors
P2-receptors (P2-Rs) are membrane proteins that lead to inhibitory or excitatory effects upon binding ADP, ATP or, in some subtypes, UTP (Bhagwat and Williams, 1997; King et al., 1998). A distinction was made between G-protein-coupled receptors and ligand-gated-ion-channel receptors as the basis for the separation of P2-Rs into two broad classes, P2Y and P2X, respectively (Abbracchio and Burnstock, 1994). P2-Rs are important targets for novel drug development for a variety of pathophysiological conditions (Chan et al., 1998; Boarder and Hourani, 1998; Barnard et al., 1997; Inoue, 1998; Abbracchio, 1996). Moreover, the large heterogeneity of P2-R subtypes in different tissues opens the possibility of developing selective organ or tissue-specific P2-R targeted drugs.
The presence of P2-Rs of the P2Y subtype on pancreatic beta cells is well documented (Loubatières-Mariani et al., 1979; Chapal and Loubatières-Mariani, 1981; Bertrand et al., 1987; Bertrand et al., 1991). The activation of pancreatic P2-Rs by extracellular ATP and structural analogues results in stimulation of insulin secretion. Structure-activity relationships of the latter analogues have been investigated (Chapal et al., 1997). The pharmacological properties and physiological relevance of P2 receptors of the insulin-secreting cell have been reviewed elsewhere (Petit et al., 1996; Loubatières-Mariani et al., 1997; Petit et al., 2001). A recent report suggests that in addition to P2Y-Rs, functional P2X-Rs are also present on pancreatic beta cells. However, whereas P2X-Rs augment insulin secretion at low, non-stimulating glucose levels, P2Y-Rs amplify insulin secretion only at stimulating glucose concentrations and do not affect, in contrast to sulfonylureas, the potassium conductance of the plasma membrane (Petit et al., 1998). The mechanism whereby P2Y-R agonists enhance glucose-induced insulin release may involve the cyclic AMP/Protein Kinase A signaling pathway (Petit et al., 2000), which has been reported to increase the effectiveness of the K+ATP channel-independent action of glucose (Yajima et al., 1999). This coupling mechanism of beta-cell P2Y receptors is supported by the glucose-dependent insulin response induced by P2Y-Rs selective ligands.
P2Y-R Ligands as Potential Antidiabetic Drugs
Various P2-R selective ligands have been shown to increase insulin secretion and decrease glycemia in vivo (Ribes et al., 1988; Hillaire-Buys et al., 1993). It was found that 2-methylthio-ATP stimulated insulin release and slightly decreased glycemia in the dog; however, to avoid its rapid breakdown into adenosine, this ATP analogue was injected directly to the pancreatico-duodenal artery (Ribes et al., 1988). Adenosine 5′-O-(2-thio)diphosphate [ADP-β-S], which is stable to enzymatic hydrolysis, was administered either intravenously or orally to rat and dog (Hillaire-Buys et al., 1993). In fed rats, ADP-β-S evoked a sustained insulin response with a reduction of glycemia. In-vivo experiments performed in conscious dogs have shown that this substance was effective after oral administration, transiently increasing insulinemia and reducing glycemia (Hillaire-Buys et al., 1993). It was also shown that the activation of P2Y-Rs was functionally effective in the pancreas of diabetic animals (Hillaire-Buys et al., 1992; Tang et al., 1996). Moreover, it was recently reported that P2Y-R activation could amplify glucose-induced insulin release from human pancreatic isolated islets (Fernandez-Alvarez et al., 2001).
Taken together, the data summarized above support the concept that P2Y-R agonists may be considered as novel insulin-releasing compounds with potential interest for the treatment of type 2 diabetes.
Identification of Potent, Stable and Subtype Selective P2Y-R Ligands
Almost all current synthetic P2-receptor agonists are modifications of the ATP or UTP pharmacophore. The purine (pyrimidine) ring system, the ribose moiety, or the triphosphate chain are modified at one or more positions (Fischer, 1999). Previously, we have reported the synthesis of potent and subtype selective P2-R-agonists (Fischer et al., 1993; Burnstock et al., 1994; Boyer et al., 1995; Boyer et al., 1996; Fischer et al., 1999). One series of these analogues represents ATP derivatives bearing a long thioether substitution at C-2 position (Fischer et al., 1993; Burnstock et al., 1994; Boyer et al., 1995; Fischer et al., 1999). Apparently, this substitution renders the molecule stable to enzymatic hydrolysis (Zimmet et al., 1993). Moreover, it increases the potency of the molecules as P2Y-Rs ligands two to five orders of magnitude compared with ATP (Fischer et al., 1993; Burnstock et al., 1994; Boyer et al., 1995; Boyer et al., 1996).
2-Thioether-5′-O-(1-thiotriphosphate) Adenosine Derivatives as Potential Insulin Secretagogues
In a previous study, we have synthesized novel P2Y-R ligands, 2-thioether-5′-O-(1-thiotriphosphate)adenosine, 2-RS-ATP-α-S, derivatives (Fischer et al., 1999), as potential insulin secretagogues. The effects of the novel analogues on insulin secretion and pancreatic flow rate were evaluated on isolated and perfused rat pancreas. A high increase, up to 500%, in glucose-induced insulin secretion was due to the addition of 2-hexylthio-ATP-α-S in the nM concentration range, which represents 100 fold enhancement of potency relative to ATP. Furthermore, these compounds are highly potent P2Y1-R-ligands in turkey erythrocytes and exhibit relative enzymatic stability regarding pancreatic type I ATPDase (Fischer et al., 1999). In addition, these compounds are highly chemically stable under physiological conditions and even under conditions simulating gastric juice acidity (Hillaire-Buys et al., 2001). However, their poor selectivity for the insulin-secreting cell, illustrated by their ability to induce vascular effects at insulin secreting concentrations, made these derivatives a priori not suitable for drug development as potential antidiabetics, since vascular events are the major pathophysiological complications of the disease.